Recently, in the field of optical communication, optical transmission schemes for realizing faster, higher capacity communication are being implemented. In addition, wavelength-division multiplexing (WDM) schemes, in which multiple optical signals at different wavelengths are multiplexed and transmitted over a single optical fiber, are becoming prevalent as a central technology for such optical transmission schemes.
Operating a stable communication system using a WDM scheme requires reserving a backup light source in case of an unexpected stop in the optical signal light source. However, in the case in which a backup light source is reserved for each wavelength of the multiplexed optical signals, the number of backup light sources increases, and there is an increase in the cost of maintaining these light sources. Accordingly, in order to moderate this cost, demand is growing for an optical module capable of modifying the wavelength of emitted light.
A representative example of such an optical module implements a technique of modifying the oscillation wavelength by varying the temperature of a semiconductor laser. In an optical module adopting this technique, the fluctuation range of the oscillation wavelength becomes at most approximately 2 nm to 3 nm, depending on the operating temperature range of the optical module. For this reason, the range of optical wavelengths that an optical module is capable of emitting is often widened by providing multiple semiconductor lasers in the optical module.
In addition, in optical modules used for optical communication, it is demanded that the wavelength of emitted light be stable over long periods. Stabilizing the wavelength requires monitoring the wavelength of emitted light, and controlling factors such as the temperature of the semiconductor laser. Accordingly, an optical module that includes the functions of a wavelength monitor has been developed (for example, see Patent Literature 1 (Unexamined Japanese Patent Application Kokai Publication No. 2002-185074) and Patent Literature 2 (Unexamined Japanese Patent Application Kokai Publication No. 2012-129259)).
The device described in Patent Literature 1 (Unexamined Japanese Patent Application Kokai Publication No. 2002-185074) uses an optical multiplexer to multiplex light emitted in the forward direction from multiple semiconductor lasers, and output the multiplexed light to optical fiber. Additionally, this device draws off part of the output light with a beam splitter or the like to monitor the wavelength.
Also, the wavelength monitor described in Patent Literature 2 (Unexamined Japanese Patent Application Kokai Publication No. 2012-129259) monitors the wavelength by causing light emitted in the backward direction from multiple semiconductor lasers disposed in an array to be collimated by a lens and incident on an etalon.
However, since the light emitted in the forward direction from the semiconductor lasers is output to optical fiber or the like and used for optical communication, with the device described in Patent Literature 1 (Unexamined Japanese Patent Application Kokai Publication No. 2002-185074), there is a risk of reduced power of the optical signal used for optical communication. Also, since the device described in Patent Literature 1 (Unexamined Japanese Patent Application Kokai Publication No. 2002-185074) is equipped with optical elements such as a beam splitter, in which there is a risk of an enlarged overall device size, and increased manufacturing costs.
On the other hand, since the wavelength monitor described in Patent Literature 2 (Unexamined Japanese Patent Application Kokai Publication No. 2012-129259) uses light emitted in the backward direction, there is no risk of reduced power in the optical signal. Also, since this wavelength monitor is not equipped with a beam splitter or the like, there is also no risk of enlarged size or increased manufacturing costs.
However, with the wavelength monitor described in Patent Literature 2 (Unexamined Japanese Patent Application Kokai Publication No. 2012-129259), from among the semiconductor lasers disposed in an array, a semiconductor laser positioned at the end emits light from a point that is greatly distanced from the center axis of the lens. For this reason, light emitted from a semiconductor laser positioned at the end enters the etalon at a large angle of incidence, and propagates in the etalon interior at a large propagation angle.
An etalon functions as a filter with periodic transmitting characteristics with respect to the frequency of light, due to the interference of multiply reflected light in the interior. For this reason, the transmittance of light that propagates at a large propagation angle is weakly frequency-dependent, and there is a risk of reduced sensitivity in the wavelength monitor.